There is growing interest in school-based programs to promote students' subjective well-being (SWB). Students with greater SWB tend to have stronger relationships with their teachers and classmates, as well as behave in more positive ways. Drawing from theory and research pertinent to promoting children's SWB, we developed an 11-session classwide positive psychology intervention that targeted elementary school students' novel use of character strengths, gratitude, kindness, and relationships in the classroom. Throughout a pilot study, children in fourth grade experienced clinically meaningful lasting gains in multiple indicators of SWB, particularly positive affect and satisfaction with self, but no changes in distal indicators of behavioral student engagement (attendance, office disciplinary referrals). This initial application by school psychologists partnering with a classroom teacher provides evidence of promise that elementary school children can benefit from participation in universal positive psychology interventions that target internal assets (gratitude, kindness, and signature strengths) and environmental resources (student-teacher and peer relationships).